Methods for making substrates and substrates formed therefrom

ABSTRACT

A method for making substrates for use in optics, electronics, or opto-electronics. The method may include transferring a seed layer onto a receiving support and depositing a useful layer onto the seed layer. The thermal expansion coefficient of the receiving support may be identical to or slightly larger than the thermal expansion coefficient of the useful layer and the thermal expansion coefficient of the seed layer may be substantially equal to the thermal expansion coefficient of the receiving support. Preferably, the nucleation layer and the intermediate support have substantially the same chemical composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 11/840,696, filedon Aug. 17, 2007, which in turn is: (1) a continuation-in-part ofapplication Ser. No. 11/505,668, filed on Aug. 16, 2006, and (2) acontinuation-in-part of application Ser. No. 10/883,437, filed on Jul.1, 2004, now U.S. Pat. No. 7,265,029, which application is (a) acontinuation of application Ser. No. 10/458,471, filed on Jun. 9, 2003,now abandoned, and (b) a continuation-in-part of application Ser. No.10/446,605, filed on May 27, 2003, now U.S. Pat. No. 6,794,276, whichapplication is a continuation of International ApplicationPCT/FR01/03714, filed on Nov. 26, 2001. The entire content of each priorapplication is expressly incorporated herein by reference thereto.

FIELD OF THE INVENTION

The present invention relates to methods for making substrates andsubstrates for use in optics, electronics or opto-electronics and, inparticular, substrates which may be used for making solar cells,light-emitting diodes and lasers.

BACKGROUND OF THE INVENTION

In the field of substrates for optics, electronics or opto-electronics,two main types of methods are well known for forming a thin layer on asupporting substrate. According to a first type of method, a thin layertaken from a donor substrate is transferred onto a receiving supportingsubstrate to obtain substrates including a thin useful layer. Usefullayer is the layer of the substrate on which electronic components suchas, for example, light-emitting diodes or other components may be made.

According to second type of method, the thin layer is deposited on areceiving supporting substrate by a deposition technique. Thisdeposition technique may notably consist of epitaxy or chemical vapordeposition. Regardless of the type of method used for forming a usefullayer on a receiving supporting substrate, in some instances it isnecessary to remove at least one portion of the receiving support toobtain a final substrate including at least the useful layer. Suchremoval of the receiving support results in loss of materials, therebyputting a strain on the manufacturing costs of such substrates.

In order to find a remedy to this drawback, a method for makingsubstrates has been devised which includes a useful thin layer method inwhich the receiving supporting substrate is removed in order to berecycled. Such a method is described in an alternative embodiment ofU.S. Pat. No. 6,794,276, which describes a method for making substrates.This method includes a step for transferring a seed layer on a receivingsupport by molecular adhesion at a bonding interface, a step for epitaxyof a useful layer on the seed layer and a step for applying stresses inorder to lead to removal of the assembly (i.e., removal of the seedlayer and of the useful layer from the receiving support at the bondinginterface). Seed layer is the material layer which allows development ofthe epitaxied useful layer.

In U.S. Pat. No. 6,794,276, certain specifications are required forallowing the seed layer to adapt to thermal expansions of the receivingsupport and the useful layer during heat treatments to which thesubstrate is subject. For this purpose, it is recommended that the seedlayer has sufficiently small thickness, of the order of 0.5 microns, andpreferably less than 1,000 Å. U.S. Pat. No. 6,794,276 also mentions thefact that the receiving support consists of a material for which thethermal expansion coefficient is 0.7 to 3 times larger than that of theuseful layer. It is specified that the thermal expansion coefficient isthe proportionality coefficient of the change in the length of a solidas a function of the initial length of the solid and of its change intemperature according to the following formula:

ΔL=αL₀ΔT where α=thermal expansion coefficient

In an alternative embodiment, the method taught by U.S. Pat. No.6,794,276 allows the receiving supporting substrate to be reused afterits removal.

It is desirable to improve the method taught by U.S. Pat. No. 6,794,276.In particular, improvements are needed for reducing the risk of breakingthe substrate, deteriorating, cracking the seed layer or the occurrenceof a residual deflection of the final substrate making it unusableduring the various heat treatments applied to the substrate. Theseimprovements are now provided by the present invention.

SUMMARY OF THE INVENTION

The invention relates to a method for making substrates for optics,electronics, or opto-electronics which includes providing a donorsubstrate and a receiving substrate, wherein the receiving substrate hasa thermal expansion coefficient; operably connecting the donor substrateto the receiving substrate; forming a seed layer on the receivingsubstrate, wherein the seed layer has a surface and a thermal expansioncoefficient; and epitaxy of a useful layer on the seed layer, whereinthe useful layer has a thermal expansion coefficient. Advantageously,the thermal expansion coefficient of the receiving substrate is equal toor greater than the thermal expansion coefficient of the useful layer,and the thermal expansion coefficient of the seed layer is about thesame as the thermal expansion coefficient of the receiving substrate sothat the seed layer and the receiving support expand in substantiallythe same way to avoid stressing or deforming the seed layer. Preferably,the nucleation layer and the intermediate support have substantially thesame chemical composition.

In another embodiment, the method for making substrates includesproviding a donor substrate and a receiving support; forming a seedlayer from the donor substrate; transferring the seed layer onto thereceiving support; and forming a useful layer on the seed layer. Again,the thermal expansion coefficient of the receiving support is equal toor greater than the thermal expansion coefficient of the useful layer,and the thermal expansion coefficient of the seed layer is about equalto the thermal expansion coefficient of the receiving support so thatthe seed layer and the receiving support expand in substantially thesame way to avoid stressing or deforming the seed layer.

Thus, during subsequent heat treatments which the structure willundergo, the seed layer and the receiving support may substantiallyexpand in the same way. The receiving support may expand slightly lessthan the seed layer so that the seed layer may be placed under slightcompression avoiding any deterioration of the seed layer.

In a preferred embodiment, the seed layer may consist of a material forwhich the thermal expansion coefficient is equal to (1+ε) times that ofthe receiving support, with ε of the order of 0.2, and preferably εequals 0.1. Further, the useful layer may consist of a material forwhich the thermal expansion coefficient may be larger than or equal to(1±ε′) times that of the receiving support, with a typical value of 0.2for ε′. The seed layer and/or the receiving support may be made of, forexample, silicon, germanium, silicon carbide, GaN or sapphire. Moreover,the chemical composition of the seed layer, advantageously, may beidentical to that of the receiving support.

A composite substrate may be created using the method described herein.The composite substrate may be used for optics, electronics, oropto-electronics. The substrate may have at least one seed layer on areceiving support, and an epitaxied useful layer on the seed layer. Thethermal expansion coefficient of the receiving support may be identicalto or slightly larger than the thermal expansion coefficient of theuseful layer, and the thermal expansion coefficient of the seed layermay be substantially equal to the thermal expansion coefficient of thereceiving support so that the seed layer and the receiving supportexpand in substantially the same way to avoid stressing or deforming theseed layer.

Other advantages and features will become better apparent from thedescription which follows of several alternative embodiments, given asnon-limiting examples, of the method for making substrates according tothe invention as well as of the substrate obtained by the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be better understood by reference to thefollowing drawings, wherein like references numerals represent likeelements. The drawings are merely exemplary to illustrate certainfeatures that may be used singularly or in combination with otherfeatures and the present invention should not be limited to theembodiments shown.

FIG. 1 is a schematic illustration of the steps of an exemplaryembodiment of a method for making a substrate; and

FIG. 2 is a schematic illustration of the steps of an alternativeexemplary embodiment of a method for making a substrate.

FIGS. 3A to 3G illustrate the successive steps of a method in accordancewith one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout this application, the term “donor substrate” is also referredto as a “source substrate.” Also, the term “receiving substrate” can beused to refer to the “intermediate support,” the “support substrate” orthe “support layer” as appropriate for the embodiment. The “seed layer”is also referred to as a “nucleation layer.” The “detachment zone” isalso referred to as a “weakened zone.” The “final support” is sometimesreferred to as a “target substrate.”

The invention relates to methods for fabricating a semiconductorsubstrate. In an implementation, the method includes providing anintermediate support 20, providing a nucleation layer 12, and providingat least one bonding layer 13 or 23 between the intermediate support 20and the nucleation layer 12 to improve the bonding energy therebetween,and to form an intermediate assembly. The technique also includesproviding at least one layer of a semiconductor material 30 upon thenucleation layer 12, bonding a target substrate 40 to the depositedsemiconductor material to form a support assembly that includes thetarget substrate 40, the deposited semiconductor material, and theintermediate assembly, and then processing the support assembly toremove the intermediate assembly. The result is a semiconductorsubstrate that includes the at least one layer of semiconductor material30 on the target substrate 40. Preferably, the intermediate assembly isconveniently removed by etching, such as with an acid solution.

Preferably, the nucleation layer 12 functions as a barrier layer againstdiffusion of atoms from the intermediate support 20 at epitaxial growthtemperatures, and the semiconductor material layer 30 is epitaxiallydeposited on the nucleation layer 12. If desired, a second barrier layermay be provided between the nucleation layer 12 and the intermediatesupport 20 prior to epitaxially depositing the semiconductor materiallayer. In an advantageous embodiment, the intermediate support 20includes a barrier layer that is resistant to diffusing elements derivedfrom dissociation of the intermediate support 20 at epitaxial growthtemperatures, and the semiconductor material is epitaxially deposited onthe nucleation layer 12. In an implementation, the barrier layer isfirst applied to the intermediate support and then the nucleation layeris applied to the barrier layer. A layer of adhesive may be applied toat least one of a surface of the barrier layer or a surface of thenucleation layer to define a bonding layer 13 or 23. Generally, at leastone of the barrier layer or the nucleation layer 12 is formed by adeposition technique.

The intermediate assembly is advantageously provided by implantingatomic species into at least a portion of a source substrate 10 todefine the nucleation layer 12, wherein a main concentration ofimplanted atomic species defines a detachment zone 14, applying the atleast one bonding layer 13 or 23 to at least one of a surface of thenucleation layer 12 or to at least a portion of a surface of theintermediate support 20, attaching the source substrate 10 implantedwith the atomic species, the at least one bonding layer 13 or 23, and atleast a portion of the intermediate support 20 together to form astructure, and treating the structure to detach the intermediateassembly from the source substrate at the detachment zone 14.

According to this aspect of the invention, the intermediate support 20is preferably selected from the group consisting of silicon, galliumarsenide, zinc oxide, lithium gallium oxide and lithium aluminum oxide.In addition, the nucleation layer 12 may include at least one of siliconcarbide, gallium nitride, or sapphire. The semiconductor material alsopreferably is made of at least one of a mono or poly-metallic nitride,and in particular gallium nitride. The nucleation layer 12 is preferablyselected from the group consisting of silicon carbide, gallium nitrideand sapphire. If desired, the final support assembly may further includea reflective coating.

Preferably, the at least one bonding layer 13 or 23 is made of at leastone of silicon oxide or silicon nitride. The target substrate 40 may bemade of at least one of monocrystalline or polycrystalline silicon.Advantageously, the final support 40 is chemically treated to remove atleast one of the intermediate support 20 or the nucleation layer 12.

Another aspect of a method for fabricating a semiconductor substrateaccording to the invention includes transferring a seed layer 12 on to asupport substrate 20, depositing a working layer 30 on the seed layer 12to form a composite substrate, and detaching the seed layer 12 and theworking layer 30 from the composite substrate to form the semiconductorsubstrate. In this implementation, the material of the seed layer 12 issuitable for accommodating the thermal expansion of the supportsubstrate 20 and the thermal expansion of the working layer 30.

Preferably, the support substrate 20 comprises a material having thermalexpansion coefficients that minimize stresses that arise duringvariations in temperature. Advantageously, the seed layer 12 istransferred onto the support substrate 20 by molecular adhesion. In animplementation, the seed layer 12 and the working layer 30 are detachedfrom the composite substrate by the application of stress at theadhesion interface, wherein the stress is selected from the groupconsisting of mechanical stress, thermal stress, electrostatic stressand laser irradiation stress, or a combination thereof.

In an implementation, the working layer 30 is made of gallium nitride.In addition, the seed layer 12 may be made of a material from the groupconsisting of sapphire, silicon carbide, zinc oxide, silicon, galliumnitride, neodymium gallate, and lithium gallate. The support substrate20 may be made of a material selected from the group consisting ofsilicon carbide, aluminum nitride, silicon, and sapphire.Advantageously, the seed layer 12 and the support substrate 20 havesubstantially the same chemical composition.

In a beneficial embodiment, the method further includes applying anintermediate layer between the seed layer 12 and the support substrate20. The intermediate layer may be at least one of a bonding layer 13 or23 or an insulating layer.

Preferably, the seed layer 12 includes a crystal lattice parametersufficient for the epitaxial growth of the working layer 30 on the seedlayer 12 such that the working layer 30 has a dislocation concentrationless than about 10⁷/cm². An advantageous implementation includesproviding a source substrate 10 including the seed layer 12 and aweakened zone 14, and detaching the seed layer 12 from the sourcesubstrate at 10 the weakened zone 14, and transfer it to theintermediate support 20. In this embodiment, the weakened zone 14includes implanted atomic species at a depth that corresponds to thethickness of the source substrate 10, and the seed layer 12 may bedetached from the source substrate 10 by application of at least one ofheat treatment, mechanical stress, chemical etching, or a combinationthereof.

The method according to this aspect of the invention preferably furtherincludes preparing the seed layer 12 to receive the working layer 30,wherein the preparation includes at least one of polishing, annealing,smoothing, oxidation, and etching. The method may also advantageouslyinclude removing the support substrate 20 such that it remains in acondition sufficient for recycling and reuse.

With reference to FIG. 1, the method according to the invention includesa step for implanting atomic species at a determined depth in a donorsubstrate 1 in order to form a weakened area 2. In step 100, the donorsubstrate may be boned upon or otherwise adhered onto a receivingsubstrate 3 by any appropriate means known in the art.

As referred to below, bonding may mean intimate contact of the donorsubstrate 1 with the receiving substrate 3 in order to join the donorsubstrate 1 and the receiving substrate 3 by molecular adhesion. Bondingmay be obtained according to various methods such as, for example, (1)having a surface of the donor substrate 1 come into direct contact witha surface of the receiving substrate; (2) forming a bonding layer inorder to make a connecting layer on the surface of the donor substrate1, forming a bonding layer in order to make a second connecting layer onthe surface of the receiving supporting substrate 3 and having thesurfaces of the respective connecting layers of the donor substrate 1and the donor substrate 3 come into contact with each other; and (3)forming a bonding layer on only one of both substrates.

In one embodiment, the bonding layer may consist of, for example, aninsulating layer or a dielectric layer. In such an embodiment, the donorsubstrate 1 may be bonded onto the receiving substrate 3 by means of abonding layer 4 deposited on the surface of the donor substrate and/orthe receiving substrate 3. In addition, an annealing step may be appliedat this stage for strengthening the bonding interface between thebonding layer 4 and the surface of the donor substrate 1 and/or thereceiving substrate 3. Nonetheless, bonding may be achieved according toany of the methods known to one skilled in the art.

In step 200, a seed layer 5 may be detached from the donor substrate 1at the weakened area 2. Thereafter, in step 300 a useful layer 6 may bedeposited on the surface of the seed layer 5. In one preferredembodiment, the useful layer 6 may be obtained by epitaxy, which is wellknown to one skilled in the art, according to step 300. The step 200 forimplanting atomic species and for detaching the seed layer 5 correspondsto a SMART-CUT® method, a general description of which is found in thepublication Silicon-On-Insulator Technology: Materials to VLSI, 2ndEdition of Jean-Pierre Colinge, Kluwer Academic Publishers, p. 50 and51. Those skilled in the art will appreciate that detachment of the seedlayer 5 and of the donor substrate 1 may be achieved by an operationsuch as, for example, heat treatment, application of mechanicalstresses, chemical etching, or a combination of at least two of theseoperations.

The seed layer 5 may consist of a material for which the thermalexpansion coefficient is equal to (1+ε) times that of the receivingsupport 3, with ε of the order of 0.2, and preferably ε equals 0.1. Itwill however be observed that thermal expansion may vary withtemperature, with the deposition technique, with the defects presentinside the layers and also with the measurement techniques. Thus, whenthe structure is undergoing heat treatments (e.g., during detachment ofthe seed layer 5 and the useful layer 6 of the receiving substrate 3)the seed layer 5 and the receiving support 3 will substantially expandin the same way. The receiving support 3 will expand slightly less thanthe seed layer 5 so that the latter may be placed under slightcompression, thereby avoiding deterioration of the seed layer 5.

The useful layer 6 may consist of a material which has a thermalexpansion coefficient which is larger than or equal to (1±ε′) times thatof the receiving support 3, with the value of ε′ between 0 and 0.8 and,preferably, between 0.2 and 0.3. Expansions of the different layers 5, 6and the receiving support 3 of the same order of magnitude during heattreatments may be obtained because of the closeness of the thermalexpansion coefficients of the useful layer 6, the seed layer 5 and thereceiving support 3. In this way, any risk of deterioration of thesubstrate or occurrence of a residual deflection of the final substratemay be avoided.

The seed layer 5 and/or the receiving support 3 may comprise a materialsuch as, for example, silicon (e.g., {111} silicon), germanium,polycrystalline or monocrystalline silicon carbide, GaN, polycrystallineor monocrystalline AlN, and sapphire. Further, the chemical compositionof the seed layer 5 may be identical with that of the receiving support3.

Between the steps for detaching 200 and for depositing 300 the usefullayer, the method may also include steps for preparing the surface ofthe seed layer 5. These preparation steps may include, for example,polishing, annealing, smooth annealing operations (e.g., underhydrogen), annealing operations for strengthening the bond, sacrificialoxidization interface operations (i.e., for oxidizing and then removingthe oxidized material), etching operations, etc.

Step 400 may lead to detachment at the bonding layer 4 of the assembly,consisting of the seed layer 5 and the useful layer 6, from thereceiving support 3. If a self-supported substrate is desired, theassembly formed by the seed layer 5 and the useful layer 6 may only beable to be detached from the receiving support 3 if the thickness of theassembly is greater than or equal to 50 μm.

In order to perform the detachment, different techniques may be used.For example, detachment may be accomplished by application ofmechanical, thermal, electrostatic stresses; application of any type ofetching (wet, dry, gas, etching, plasma etching, etc.) and/orapplication of any type of etching by irradiation such as laserirradiation (e.g., by chemical etchings at the bonding layer 4), or thelike. The receiving substrate 3, which may either be destroyed orrecycled in order to reuse it during the making of a new substrate, maythen be obtained on the one hand, and a structure consisting of the seedlayer 5 and the useful layer 6 may be obtained on the other hand. Itwill be appreciated that for performing the detachment of the assembly(consisting of the seed layer 5 and the useful layer 6) from thereceiving support 3 at the bonding layer 4, chemical etching mayadvantageously be used if the receiving substrate 3 is intended to bedestroyed. On the other hand, if the receiving substrate 3 is intendedto be recycled for reuse, mechanical stress or chemical etching of thebonding layer 4 may preferably be used, which provides full detachmentof substrate 3. The seed layer 5 may then be removed by any appropriatemeans known to those skilled in the art.

Thereafter, the useful layer 6 may be transferred onto a finalsupporting substrate 7. The final support 7 may be made of a materialsuch as, for example, semi-conducting or semi-conductive materials(e.g., silicon, germanium, etc.), metals (e.g., copper), plasticmaterials and glasses. Since the resultant structure no longer undergoesany heat treatment, the final supporting substrate 7 may be made withany material which has a thermal expansion coefficient and/or a latticeparameter different from those of the useful layer 6.

In a preferred embodiment, the useful layer 6 may be transferred ontothe final supporting substrate 7 by bonding. The bond may be obtained byapplying a bonding layer 8 on one of the surfaces of the useful layer 6and/or the final supporting substrate 7. Similar to selecting the finalsubstrate 7, the bonding techniques applied in this step are not limitedby temperature resistance, contaminations, the thermal expansioncoefficient and/or the lattice parameter of the useful layer 6.

The layer 8 used may comprise, for example, organic layers (e.g.,insulating layers of the SiO₂, Si₃N₄, or polyimides), conductive metalinterfaces and seals (e.g., palladium silicide Pd₂Si, tungsten silicideWSi₂, SiAu, or PdIn). The conductive interfaces may then provide thecontact on the rear face of the layer.

Moreover, structures may be buried in this bonding layer 8 so that arear junction contact of a triple junction may thereby be made forproducing solar cells. In one embodiment, the buried structure mayconsist of a triple junction based on amorphous silicon of the n-i-ptype. This buried structure may have a lower layer (i.e., a rear contactlayer) consisting of metallization, such as silver (Ag) or aluminium(Al), on which a conducting transparent oxide may be deposited. The rearcontact layer, on the one hand, may provide an electrical contact withwhich the triple junction solar cell may be connected and a rear mirror,on the other hand, allowing reflection of light which has not beenabsorbed by the solar cell. The latter may consist of three amorphoussilicon layers (of type n, i and p, respectively) successively depositedon the rear contact layer. It will be appreciated by those skilled inthat art that when making LEDs, mirrors may also be buried in thebonding layer 8.

In an alternative embodiment (not illustrated in FIG. 1), the usefullayer 6 and the seed layer 5 may be transferred onto the finalsupporting substrate 7 with or without the bonding layer 8 prior toremoving the seed layer 5.

Referring now to FIG. 2, atomic species may be implanted in the same wayas previously discussed—at a determined depth of a donor substrate 1—inorder to form a weakened area 2. The donor substrate 1 in step 100 maythen be adhered on a receiving substrate 3 by any appropriate means. Instep 200, a seed layer 5 may be detached from the donor substrate 1 atthe weakened area 2. Thereafter, in step 300, a useful layer 6 may bedeposited on the surface of the seed layer 5. Detachment of the seedlayer 5 and the donor substrate 1 may be achieved by an operation suchas, for example, heat treatment, application of mechanical stresses andchemical etching, or a combination of at least two of these operations.

In another alternative embodiment, the seed layer 5 may originate fromthe thinning of the donor substrate (for example according to a BESOItype method) before depositing the useful layer 6. The final supportingsubstrate 7 may then be transferred onto the useful layer 6 by means ofa bonding layer 8. Stresses may be applied in order detach thestructure, which may consist of the seed layer 5, the useful layer 6,the bonding layer 8 and the final supporting substrate 7, from thereceiving support 3 at the bonding layer 4. A receiving substrate 3,ready to be recycled, may be obtained on the one hand and a structureconsisting of the seed layer 5, the useful layer 6, the bonding layer 8and the final supporting substrate 7 may be obtained on the other hand.The seed layer 5 may then be removed by any appropriate means in orderto obtain the final substrate.

EXAMPLES

Two particular but non-limiting exemplary embodiments of a resultantsubstrate will be described hereafter with reference to FIG. 2. Thesubstrates are intended for making solar cells (Example 1) andlight-emitting diodes (Example 2). It should be noted, however, that theexamples are not intended to be limiting as to the fields of applicationof the invention.

Example 1

According to this example, a weakened area 2 may be made by implantingatomic species at a determined depth in the donor substrate 1 which maybe made of, for example, germanium (Ge). The receiving substrate 3,which may also be made of Ge, may be bonded to the donor substrate 1 bymeans of a bonding layer 4. The bonding layer 4, preferably made ofnitride or oxide, may be formed on the face of at least one of the donor1 or receiving 3 substrates.

As shown in step 200, a seed layer 5 of Ge may be detached from thedonor substrate 1 at the weakened area 2 using the SMART-CUT® method asdescribed herein. The seed layer 5 of Ge may have a thermal expansioncoefficient (which is also noted as CTE) which varies from 4.6 to 6.6710⁻⁶ for temperatures ranging from 25° C. to 600° C. Detachment of theseed layer 5 and the donor substrate 1 may be achieved by an operationsuch as, for example, heat treatment, application of mechanical stressesand chemical etching, or a combination of at least two of theseoperations.

As illustrated in step 300, a useful gallium arsenide layer 6 may thenbe deposited on the surface of the seed layer 5. The CTE of AsGa may befrom 5.00 to 7.4 10⁻⁶ for temperatures ranging from 25° C. to 600° C.Different layers, such as, for example, InP, AsGa, GaInP, InGaAs,InGaAlP, or InGaAsN epitaxied layers, may be successively deposited byepitaxy on the deposit of the AsGa layer in order to form an epitaxialstack for making junctions (e.g., triple junctions, quadruple junctions,etc.). It will be appreciated that the useful layer 6 may have acrystalline quality at least equal to the crystalline quality which maybe obtained by epitaxy on a massive Ge substrate.

The useful layer 6 and the seed layer 5 may then be transferred onto afinal supporting substrate 7. It will be noted that the final support 7may also be contacted with the epitaxial stack if the latter is madebeforehand. The final support 7 may be made of a material such as, forexample, semi-conductors (e.g., silicon, germanium), plastic materialsand glasses. Transfer of the useful layer 6 and the seed layer 5 ontothe final supporting substrate 7 may be performed by bonding. The bondmay be performed using a bonding layer 8 made of, for example,insulating layers (e.g., SiO₂, Si₃N₄, etc.), organic layers (e.g.,polyimides), metal layers (e.g., palladium silicide Pd₂Si and tungstensilicide WSi₂), and seals (e.g., SiAu, PdIn, etc.).

The final supporting substrate 7, the seed layer 5 and the useful layer6 may then be detached by any appropriate means, for example, at thebonding layer 4 from the receiving support 3. The receiving support 3may thereafter be recycled advantageously. This detachment may beobtained by applying stresses at the bonding interface such as, forexample, mechanical stresses, thermal stresses, electrostatic stressesand stresses from laser irradiation. Thereafter, the seed layer may beremoved in order to obtain the final substrate consisting of the finalsupporting substrate 7 of the bonding layer 8 and the useful layer 6 orthe epitaxial stack.

Example 2

A weakened area 2 may be achieved by implanting atomic species at adetermined depth in the donor substrate 1 made of massive sapphire whichhas a CTE that varies from 4 to 9.03 10⁻⁶ for temperatures ranging from25° C. to 1,000° C. The implantation may consists of implanting hydrogenat a dose between 0.5×10¹⁷ and 3×10¹⁷ at/cm², preferably, between 1 and2×10¹⁷ at/cm² and an energy of the order of 20 to 210 keV, preferably100 keV. The receiving substrate 3 also may also be made of massivesapphire and may be adhered to the substrate 1 by means of a bondinglayer 4. The bonding layer 4 may reach a thickness of one micron whichmay thereby facilitate subsequent detachment of the receiving supportingsubstrate 3 by side chemical etching of this bonding layer 4.

As shown in step 200, a seed layer 5 of sapphire may be detached fromthe donor substrate 1 at the weakened area 2 by following the SMART-CUT®method. Thereafter, in step 300, useful layers 6 may be deposited on thesurface of the seed layer 5. The useful layers 6 may be deposited byepitaxy based on, for example, GaN, AlN, InGaN, InN and their ternarycompounds (AlGaN, InGaN).

Detachment of the seed layer 5 and the donor substrate 1 may be achievedby an operation such as, for example, heat treatment, application ofmechanical stresses and chemical etching, or a combination of at leasttwo of these operations. The useful layer 6 and the seed layer 5 maythen be transferred onto a final supporting substrate 7 of sapphire or amaterial such as, for example, silicon, copper, plastic materials, andglass. This transfer of the useful layer 6 and the seed layer 5 onto thefinal supporting substrate 7 may be performed by bonding. The bond beingobtained by using a bonding layer made of, for example, insulatinglayers (e.g., SiO₂, Si₃N₄, etc), organic layers (e.g., polyimides),metal layers (e.g., palladium silicide Pd₂Si and tungsten silicide WSi₂)or seals (e.g., SiAu, PdIn).

The final supporting substrate 7, the seed layer 5 and the useful layer6 may then be detached by any appropriate means, at the bonding layer 4,from the receiving support 3. The receiving support 3 may advantageouslybe recycled.

While the foregoing description and drawings represent the preferredembodiments of the present invention, it will be understood that variousadditions, modifications and substitutions may be made therein withoutdeparting from the spirit and scope of the present invention as definedin the accompanying claims. In particular, it will be clear to thoseskilled in the art that the present invention may be embodied in otherspecific forms, structures, arrangements, proportions, and with otherelements, materials, and components, without departing from the spiritor essential characteristics thereof. One skilled in the art willappreciate that the invention may be used with many modifications ofstructure, arrangement, proportions, materials, and components andotherwise, used in the practice of the invention, which are particularlyadapted to specific environments and operative requirements withoutdeparting from the principles of the present invention. The presentlydisclosed embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims, and not limited to the foregoingdescription.

1. A substrate comprising: a receiving support having a thermalexpansion coefficient; a seed layer having a thermal expansioncoefficient, wherein the seed layer is operably connected to thereceiving support; and a useful layer having a thermal expansioncoefficient, the useful layer being operably connected to the seedlayer; wherein the thermal expansion coefficient of the receivingsupport is greater than or equal to the thermal expansion coefficient ofthe useful layer, and wherein the thermal expansion coefficient of theseed layer is about equal to the thermal expansion coefficient of thereceiving support so that the seed layer and the receiving supportexpand in substantially the same way to avoid stressing or deforming theseed layer.
 2. The substrate of claim 1, wherein the seed layer is madeof a material for which the thermal expansion coefficient is equal to(1±′) times the thermal expansion coefficient of the receiving support.3. The substrate of claim 1, wherein the useful layer is made of amaterial for which the thermal expansion coefficient is greater than orequal to (1±′) times the thermal expansion coefficient of the receivingsupport.
 4. The substrate of claim 1, wherein the at least one of theseed layer and the receiving support is made of a material selected fromthe group consisting of silicon, germanium, silicon carbide, GaN, AlNand sapphire and optionally where the chemical composition of the seedlayer and that of the receiving substrate are identical.
 5. Thesubstrate of claim 1, further comprising a supporting substratecomprising a material selected from the group consisting ofsemiconductors, plastic, glass and metal, and optionally including abonding layer connecting the supporting substrate and the useful layer.6. The substrate of claim 5, further comprising a structure buried inthe bonding layer.
 7. The substrate of claim 1, further comprising abonding layer connecting the seed layer and the receiving support,wherein the bonding layer is comprised of a material selected from thegroup consisting of insulating layers, organic layers, metal interfacesand sealing layers.
 8. The substrate of claim 1, wherein the bondinglayer comprises at least one of silicon oxide or silicon nitride.
 9. Thesubstrate of claim 1, wherein the seed layer and the useful layer has athickness of at least 50 μm.